The quantitative distribution of the major types of hypophyseal secretory cells have been determined in nine men and five women dying without known endocrine disease. Based on this study the mean prolactin cell population is 3.6 percent, and the upper limit observed in 5.5 percent. A very high prevalence of prolactin cell hyperplasia has been discovered in hypophyseal histologic sections from 154 men and 142 women. Twenty-five percent of all glands showed diffuse prolactin cell proliferation. Fifty-seven percent had known factors causing hyperprolactinemia: including oral contraceptives, estrogens, phenothiazines, and pregnancy. Men less than sixty years developed prolactin cell hyperplasia as commonly as women; the prevalence after sixty was increased in men over women. Model studies using estrogen treated male rats and male and female rats two years old showed that prolactin adenoma formation is preceded by a stage of diffuse prolactin cell hyperplasia and hyperprolactinemia. Estradiol has been found to markedly inhibit growth and hormone secretion by the MtTW15 rat pituitary tumor. Four cell lines derived from this tumor are being used to explore estrogen sensitivity in vitro. Peptide hormones are being localized in normal and neoplastic pituitary cells of rat and man using ultrastructural immunocytochemistry.